Enriched end-member of primitive MORB melts: petrology and geochemistry ofglasses from Macquarie Island (SW Pacific)

Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia-Pacific plate boundary sout h of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transfo rm faults linked by short spreading segments, and currently into a right-la teral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intr usive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyk e complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias , hyaloclastites), and associated sediments. A set of Macquarie Island basa ltic glasses has been analysed by electron microprobe for major elements, S , Cl and F; by Fourier transform infrared spectroscopy for H2O; by laser ab lation-inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstand ing compositional feature of the data set (47.4-51.1 wt % SiO2, 5.65-8.75 w t % MgO) is the broad range of K2O (0.1-1.8 wt %) and the strong positive c ovariation of K2O with other incompatible minor and trace elements (e.g TiO 2 0.97-2.1%; Na2O 2.4-4.3%; P2O5 0.08-0.7%; H2O 0.25-1.5%; La 4.3-46.6 ppm) . The extent of enrichment in incompatible elements in glasses correlates p ositively with isotopic rations of Sr (Sr-87/Sr-86 = 0.70255-0.70275) and P b (Pb-206/Pb-204 = 18.951-19.493; Pb-207/Pb-204 = 15.528-15.589; Pb-208/Pb- 204 = 38.523-38.979), and negatively with Nd (Nd-143/Nd-144 = 0.51310-0.513 04). Macquarie Island basaltic glasses are divided into two compositional g roups according to their mg-number-K2O relationships. Near-primitive basalt ic glasses (Group I) have the highest mg-number (63-69), and high Al2O3 and CaO contents at a given K2O content, and carry microphenocrysts of primiti ve olivine (Fo(86-89.5)). Their bulk compositions are used to calculate pri mary melt compositions in equilibrium with the most magnesian Macquarie Isl and olivines (Fo(90.5)). Fractionated, Group II, basaltic glasses are satur ated with olivine + plagioclase +/- clinopyroxene, and have lower mg-number (57-67), and relatively low Al2O3 and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions t hat represent a new primitive enriched MORB end-member. Compared with N-MOR B, this new end-member is characterized by relatively low contents of MgO, FeO, SiO2 and CaO, coupled with high contents of Al2O3, TiO2, Na2O, P2O5, K 2O and incompatible trace elements, and has the most radiogenic Sr and Pb r egional isotope composition. These unusual melt compositions could have bee n generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magm as. The mantle in the Macquarie Island region must have been enriched and h eterogeneous on a very fine scale. We suggest that the mantle enrichment im plicated in this study is more likely to be a regional signature that is sh ared by the Balleny Islands magmatism than directly related to the hypothet ical Balleny plume itself.